Method for using and transporting a non-emulsified organic peroxide-containing composition

ABSTRACT

A method of using an organic peroxide containing composition as a gel breaker and a method of transporting the composition is disclosed, wherein the composition has a freeze point less than or equal to −10° C. and a diluent which contains water and an organic solvent selected from ethylene glycol, methanol, ethanol and 1-propanol. During shipment, the amount of available oxygen in the composition is less than or equal to 1 weight percent.

FIELD OF THE INVENTION

The invention relates to a method of using and transporting anon-emulsified well treating composition.

BACKGROUND OF THE INVENTION

In hydraulic fracturing, a fracturing fluid is injected into a wellboreunder high pressure. Once the natural reservoir pressures are exceeded,the fracturing fluid initiates a fracture in the formation. The fractureusually continues to grow during pumping. Typically, treatment designrequires the fracturing fluid to reach maximum viscosity as it entersthe fracture since this affects fracture length and width. Thefracturing fluid may include a proppant; the proppant being placedwithin the produced fracture. The proppant remains in the producedfracture to prevent the complete closure of the fracture and to form aconductive channel extending from the wellbore into the formation.

Viscosity affects the fluid's ability to place proppant within theproduced fracture. In addition, fluid viscosity influences fracturegeometry and minimizes fluid loss. The fracturing fluid's viscosity mayprincipally be attributed to the presence of polymers, such aspolysaccharides, in the fracturing fluid. To further enhance theviscosity, a crosslinking agent is frequently added to the fracturingfluid to gel the polymer.

The recovery of fracturing fluid from the formation is accomplished byreducing the viscosity of the fluid. Such reduction in viscosity shouldfurther be instrumental in retention of proppant within the fracture.Viscosity reduction may be accomplished by incorporating breakers intothe initial fracturing fluid.

Many chemical agents have been reported in the literature for use asbreakers. Conventional breakers have included oxidizers, acids andenzymes. In light of their reactivity and oxidative capacity, peroxideshave been used as breakers for many years. For instance, U.S. Pat. No.5,447,199 reports the use of organic peroxides, slightly soluble inwater (less than about 5% solubility), with water immisciblenon-oxidizable organic solvents. Such peroxides render controlledviscosity reduction rate to the fracturing fluid after the fluid ispumped into the formation. The viscosity is controlled by the diffusionof the peroxide out of the oil phase and by the slow reaction of theperoxide and polymer. This allows for efficient recovery of thefracturing fluid from the formation.

In many instances, however, it is more desirable to use a water-solubleperoxide as the breaker composition. U.S. Pat. No. 3,922,173 disclosesthe use of t-butyl hydroperoxide in a gelled fluid as a breaker.However, no teaching is provided on the transport, storage and/or use ofthe product in cold weather environments.

In order for peroxides to be suitable at sub-freezing conditions, thebreaker composition must exhibit a low freeze point. Dilution of watersoluble organic peroxides with only water as diluent is unsuitable sincethe resulting composition is unacceptable for use at sub-freezingconditions.

Further, the difficulty in using organic peroxides in breakercompositions is compounded by stringent storage and transportregulations. Peroxides are typically prone to violent decompositioninitiated by such external factors as mechanical agitation, frictionand/or heat. Thus, handling and storage regulations as well as limits oncontainer size have evolved in regards to the transport of suchcompounds. Such regulations have been implemented to reduce the risk ofexplosion which may, in turn, be caused by oxidation of peroxides in thepresence of atmospheric oxygen.

A review of governmental transport and storage regulations appears in“Recommendations on the Transport of Dangerous Goods” Vol. 1, 14^(th)revised edition, United Nations, New York and Geneva, 2005. While theregulations state that select organic peroxides may be diluted with a“suitable solvent”, defined as a “Type A solvent” having a boiling pointgreater than 150° C. and which is compatible with the organic peroxide,no identification of a “suitable solvent” or a “Type A” is made. Thus,extensive testing is required in order to identify an acceptablesolvent. Further, the regulations mandate a restricted container sizeand extensive explosion testing must be undertaken in such containers tosatisfy the stated criteria. Further, the diluted product must be storedin an isolated and controlled area.

It therefore is desired to develop peroxide-containing breakercompositions having a freeze point which is acceptable for use in coldenvironments. It is also desired to develop peroxide-containing breakercompositions which may be stored and transported to their destination ofuse and thus used on-the-fly, wherein such breaker compositions exhibitsa low decomposition rate. Furthermore, it is desired to developperoxide-containing breaker compositions which would render unnecessarythe need for explosion testing of shipping containers containing thecompositions and allow the transport of the breaker without restrictionson container size.

SUMMARY OF THE INVENTION

The organic peroxide containing composition used herein is outside ofcurrent governmental regulations relating to the transportation andstorage of organic peroxides. The organic peroxide containingcomposition is a dilute composition of water and at least one watermiscible solvent. The available oxygen content in the composition isless than 1 weight percent. Reduced safety hazards make the dilutionprocess a very simple and convenient method of using organic peroxidesas breakers for water based fracturing fluids. [Even in its dilutedstate (less than 1 wt % oxygen), the rate of addition of the dilutedproduct to the fracturing fluid is typically less than 2%.]

Since the organic peroxide containing composition is typically used inlocations where ambient temperature conditions are less than 0° C., thediluent typically contains a suitable freeze point depressing solvent.The choice of freeze point depressing solvent may be made based on suchfactors as cost, effect on degradation rate of the peroxide, anticipatedshelf life, degree of freeze point depression required and the effect ofthe solvent on the overall safety hazard of the diluted product.Typically, the breaker composition requires a freeze point which is atleast less than or equal to −10° C. In preferred embodiments, the freezepoint may be less than or equal to −20° C. and in many cases less thanor equal to −40° C. The breaker composition exhibits an available oxygencontent of less than 1 weight percent and principally exists in anon-emulsified state.

The organic peroxide is substantially soluble in water. The diluentcontains water and a water miscible solvent selected from ethyleneglycol, methanol, acetone, ethanol and 1-propanol.

The amount of organic solvent in the breaker composition is between fromabout 0 to about 95 volume percent, preferably between from about 30 toabout 70 volume percent.

In a preferred embodiment, the organic peroxide is t-butyl hydroperoxideand the organic solvent is ethylene glycol.

The concentration of organic peroxide in the breaker composition is suchthat the amount of available oxygen in the breaker composition is lessthan or equal to 1 weight percent. At such concentrations, explosiontesting of shipping containers holding the compositions is unnecessaryunder regulations of certain jurisdictions.

The breaker compositions have a degradation of less than 20% over theexpected storage life and storage temperature of the product.

The breaker compositions are extremely useful in the degradation of gelsin subterranean formations. As such, the breaker compositions may beintroduced into the formation with a fracturing fluid of an aqueousfluid and hydratable polymer and then pumped to a desired locationwithin the wellbore under pressure sufficient to fracture thesubterranean formation. The breaker is then capable of degrading thepolymer to render a pumpable fluid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The breaker compositions for use in the method defined herein contain anorganic peroxide and a diluent. The peroxide in the composition isdilutes such that less than 1 weight percent active oxygen is in thebreaker composition.

The diluent in the breaker composition remains liquid and is stable atsubfreezing conditions. As such, the breaker compositions haveparticular applicability for use in well treatment servicing whenconditions are below freezing.

In particular, the use of the organic solvent provides excellentshelf-life freeze-proofing to the composition. For instance, the freezepoint temperature of the breaker composition may be less than or equalto −10° C. In some cases, the freeze point of the breaker composition isless than or equal to −20° C. In other cases, the freeze point of thebreaker composition is less than or equal to −30° C. Still, in othercases, the freeze point of the breaker composition is less than or equalto −40° C. As such, the breaker composition exhibits excellent storagecapabilities at very low temperatures.

The shelf-life of the breaker compositions defined herein is typicallyin excess of three months when stored or transported at a temperature of25° C. or lower.

The breaker composition has an active oxygen content equal of less than1 weight % and principally exists in a non-emulsified state.

The diluent contains water and/or an organic solvent acceptable forfreeze-proofing. The organic solvent must be compatible with the organicperoxide. Compatibility may be measured by assaying the activity of theperoxide during storage. Preferably, the activity of the peroxide willnot deteriorate by less than 20% over a storage period of three months.Preferred organic solvents are those selected from the group consistingof ethylene glycol, methanol, ethanol, 1-propanol and acetone. Ethyleneglycol is most preferred since it has been illustrated to provide veryhigh stability to the resulting composition. Further, low molecularweight alcohols, as well as acetones, are generally less preferred inlight of domestic and/or international shipping regulations.

The organic peroxide is substantially soluble in water so as to remainas a single homogeneous phase in the diluent. “Substantially soluble,”is meant to mean up to a solubility of at least 10 weight %. Further, inorder to conserve costs, the peroxide should be of relatively lowmolecular weight, typically less than or equal to approximately 200, inorder to maximize available oxygen content.

Exemplary organic peroxides include t-butyl hydroperoxide, disuccinicacid peroxide, dipropionyl peroxide, diacetone alcohol peroxides,di-(2-methylbenzoyl) peroxide, and 3-chloroperoxybenzoic acid. Of these,t-butyl hydroperoxide is especially preferred. Commercially availablet-butyl hydroperoxides are typically about 70% active in water and canbe readily diluted in the organic solvents described herein. Suchcommercially available hydroperoxides include Trigonox A-W 70 of AKZONobel.

The amount of organic solvent in the breaker composition is between fromabout 0 to about 95 volume percent, preferably between from about 30 toabout 70 volume percent.

In a preferred embodiment, the organic peroxide is t-butyl hydroperoxideand the organic solvent is ethylene glycol.

The maximum amount of organic peroxide employed in the breakercomposition is that required to provide an available oxygen content of 1weight percent. As such, the breaker compositions of the invention arepreferably non-emulsified compositions At such concentrations, thebreaker composition may be transported in shipping containers withoutthe need for conducting explosion tests on representative containers,under the regulations of some jurisdictions.

In addition to having low freeze points, the breaker compositionsdefined herein exhibit high flash points. Typically, the flash point ofthe breaker compositions is in excess of 37.8°.

Further, the breaker compositions display excellent compatibility withelastomers in conventionally employed pumping equipment, are readilymiscible in water and demonstrate no adverse facts on crosslinked watergels.

In addition to exhibiting an acceptable freeze point, the breakercompositions for use herein do not exhibit separation into liquid phasesnor does the precipitation of peroxide crystals occur at subfreezingconditions.

The breaker compositions are extremely useful in the degradation of gelsin subterranean formations. As such, the breaker composition may beintroduced into the formation with a fracturing fluid of an aqueousfluid, hydratable polymer and optional crosslinking agent. The aqueousfluid could be, for example, water, brine, aqueous based foams as wellas mixtures of water and alcohol. Alternatively, the fracturing fluidintroduced into the formation may by prepared by combining hydratablepolymer, aqueous fluid, optional crosslinking agent with the organicperoxide and diluent described herein. Any suitable mixing apparatus maybe used for this procedure.

The fluid may then be pumped to a desired location within the wellboreunder pressure sufficient to fracture the surrounding subterraneanformation. The breaker, as defined herein, degrades the polymer, wherebythe fluid may be pumped from the subterranean formation to the wellsurface.

The hydratable polymer may be any of the hydratable polysaccharides thatare conventionally employed in the well service industry. Thesepolysaccharides may be capable of gelling in the presence of acrosslinking agent to form a gelled based fluid. For instance, suitablehydratable polysaccharides are the galactomannan gums, guars,derivatized guars, cellulose and cellulose derivatives. Specificexamples are guar gum, locust bean gum, fenugreek gum, caraya gum,xanthan gum, cellulose, and derivatives of these gums. The preferredgelling agents are guar gum, carboxymethyl guar, hydroxypropyl guar,carboxymethyl hydroxypropyl guar, cellulose, carboxymethyl cellulose,carboxymethyl hydroxyethyl cellulose and hydroxyethyl cellulose. Themost preferred gelling agents are guar gum, hydroxypropyl guar,carboxymethyl hydroxypropyl guar, hydroxyethyl cellulose andcarboxymethyl hydroxyethyl cellulose.

The polysaccharide of the fracturing fluid may further not becrosslinked. For instance, polysaccharides not requiring a crosslinkingagent, such as starch, derivatized starch, xanthans and xanthan gums,may be used.

The fracturing fluids of the invention often include the crosslinkingagent. The crosslinking agent can be any of the conventionally usedcrosslinking agents which are known to those skilled in the art. Forinstance, in recent years, gellation of the hydratable polymer has beenachieved by crosslinking these polymers with metal ions includingaluminum, antimony, zirconium, for example, zirconium chelates such aszirconium acetate, zirconium lactate, zirconium lactate triethanolamineand titanium containing compounds including organotitinates, forexample, the titanium chelates such as triethanolamine titanates,titanium acetylacetonate and titanium lactate. See, for instance, U.S.Pat. No. 4,514,309.

Borate crosslinkers are further typically used. Such crosslinking agentsare those capable of supplying borate ions in solution. For instance,such crosslinkers include those which are a convenient source of borateions, for instance the alkali metal and the alkaline earth metal boratesand boric acid. One such crosslinking additive is sodium boratedecahydrate, the crosslinking agent described in U.S. Pat. No.5,160,643. In a preferred embodiment of the invention, the fracturingfluid contains guar and a borate crosslinking agent. In such guar gels,the crosslinking additive is preferably present in the range from about0.024% to in excess of 0.18% by weight of the fracturing fluid.Preferably, the concentration of crosslinking agent is in the range fromabout 0.024% to about 0.09% by weight of the fracturing fluid.

Any proppant known in the art may be added to the fracturing fluid. If acrosslinking agent is present in the fracturing fluid, the proppant istypically added to the fluid prior to the addition of the crosslinkingagent. Suitable proppants include quartz sand grains, glass, ceramics,walnut shell fragments, aluminum pellets, nylon pellets and the like.The proppants are normally used in concentrations between about 1 to 18pounds per gallon of fracturing fluid composition, but higher or lowerconcentrations can be used as required.

The fracturing fluid may also contain other conventional additivescommon to the well service industry such as surfactants and the like.

The following example illustrates the practice of the present inventionin a preferred embodiment. Other embodiments within the scope of theclaims herein will be apparent to one skilled in the art fromconsideration of the specification and practice of the invention asdisclosed herein. It is intended that the specification, together withthe example, be considered exemplary only, with the scope and spirit ofthe invention being indicated by the claims which follow.

EXAMPLES Example 1

A solution of 8.5 volume % t-butyl hydroperoxide (70% in water) wasprepared in a blend of 49.0 volume percent water and 43.5 volume percentethylene glycol. The solution exhibited a flash point of 69.4° C. and afreeze point of −40° C. After storage at room temperature for 4 monthsthe t-butyl hydroperoxide content was analyzed by a gas chromatographequipped with an mass selective detector and the concentration was foundto have degraded less than 5% compared to a new solution. Thisdemonstrates the suitability of the ethylene glycol as a diluent in thisinvention.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the true spirit andscope of the novel concepts of the invention.

1. A method of degrading a gel in a subterranean formation whichcomprises introducing into the formation a non-emulsified well treatingcomposition comprising an organic peroxide and an organic solventselected from the group consisting of ethylene glycol, acetone,methanol, ethanol and 1-propanol, wherein the well treating compositionhas a freeze point less than or equal to −10° C.
 2. The method of claim1, wherein the well treating composition has a freeze point less than orequal to −20° C.
 3. The method of claim 1, wherein the organic peroxideis t-butyl hydroperoxide.
 4. The method of claim 1, wherein the organicsolvent is ethylene glycol.
 5. The method of claim 4, wherein the amountof ethylene glycol in the well treating composition is between fromabout 10 to about 95 volume percent.
 6. The method of claim 1, whereinthe amount of organic solvent in the breaker composition is between fromabout 10 to about 95 volume percent.
 7. The method of claim 6, whereinthe amount of organic solvent in the breaker composition is between fromabout 30 to about 70 volume percent.
 8. The method of claim 1, whereinthe composition has a self accelerating decomposition temperaturegreater than or equal to 55° C.
 9. The method of claim 1, wherein theamount of available oxygen in the composition is less than or equal to 1weight percent.
 10. A method of fracturing a subterranean formationsurrounding a wellbore which comprises the steps of: (a) introducinginto the formation a fracturing fluid of an aqueous fluid, a hydratablepolymer and a breaker; (b) pumping the fracturing fluid to a desiredlocation within the wellbore under pressure sufficient to fracture thesurrounding subterranean formation; and (c) allowing the breaker todegrade the polymer and produce a pumpable fluid wherein the breaker isa composition comprising an organic peroxide which exhibits substantialsolubility in water and a diluent selected from the group consisting ofwater, ethylene glycol, acetone, methanol, ethanol and 1-propanol. 11.The method of claim 10, wherein the organic peroxide is t-butylhydroperoxide.
 12. The method of claim 11, wherein the organic solventis ethylene glycol.
 13. A method of transporting an organic peroxidewhich exhibits substantial solubility in water, the method comprising:(a) preparing a dilute non-emulsified peroxide solution by mixing anorganic peroxide with a sufficient amount of diluent in order to rendera available oxygen content in the dilute peroxide solution in an amountless than or equal to 1 weight percent, wherein the diluent is a mixtureof water and an organic solvent selected from the group consisting ofethylene glycol, acetone, methanol, ethanol and 1-propanol; and (b)transporting a receptacle containing the dilute peroxide solution to adesired location.
 14. The method of claim 13, wherein the activity ofthe peroxide will not deteriorate by less than 20% over a storage periodof three months.
 15. The method of claim 13, wherein the freeze point ofthe dilute peroxide solution is less than or equal to −10° C.
 16. Themethod of claim 15, wherein the freeze point of the dilute peroxidesolution is less than or equal to −20° C.
 17. The method of claim 16,wherein the freeze point of the dilute peroxide solution is less than orequal to −40° C.
 18. The method of claim 13, wherein the organicperoxide is t-butyl hydroperoxide.
 19. The method of claim 18, whereinthe organic solvent is ethylene glycol.
 20. The method of claim 13,wherein the amount of organic solvent in the dilute peroxide solution isbetween from about 10 to about 95 volume percent.
 21. In a method oftransporting an aqueous organic peroxide solution in a shippingcontainer, the improvement comprising shipping a dilute non-emulsifiedorganic peroxide solution wherein the amount of available oxygen in thedilute organic peroxide solution is less than or equal to 1 volumepercent, the diluent being water and an organic solvent selected fromthe group consisting of ethylene glycol, acetone, methanol, ethanol and1-propanol.
 22. The method of claim 21, wherein the freeze point of thedilute organic peroxide solution is less than or equal to −20° C.